In the subtractive color process, a color image, as is well known, can be formed by the oxidative coupling reaction that takes place in a silver halide emulsion between a yellow-dye, a cyan-dye- or a magenta-dye-forming coupler and the oxidation product of a color developing agent of an aromatic primary amine type, which is produced in reduction of silver halide grains exposed to light by the color developing agent.
In the above process, compounds having an active methylene group can be generally employed as a yellow coupler for forming a yellow dye, compounds of pyrazolone, pyrazolobenzimidazole, indazolone or like types can be generally employed as a magenta coupler for forming a magenta dye-, and compounds having a phenolic or naphtholic OH group can be generally employed as a cyan coupler for forming a cyan dye.
Each coupler is dissolved in a high boiling organic solvent insoluble substantially in water, and then added to a silver halide emulsion, which high boiling organic solvent may optionally be used together with auxiliary solvents. Also, each coupler may be added to an emulsion in the form of an alkaline aqueous solution. The former emulsion can produce a dye image superior to the latter emulsion in light fastness, moisture resistance, heat resistance, graininess and color sharpness.
Essential properties required of each coupler include not only forming a dye, but also having high solubility in a high boiling organic solvent or an alkaline aqueous solution, and having good dispersibility and stability in a silver halide photographic emulsion. In addition, it is to be desired that a coupler should produce such a dye as to be excellent in fastness to light, heat and moisture, spectral absorption characteristics, and transparency; provide such a developed image as to have distinctness and, what is of greater importance, high color density; have a high dye-forming speed; and so on.
In recent years, color photographic material makers have made it feasible to take photographs at night, and to take photographs of scenes which necessitate a high speed shutter, e.g., to take photographs of sporting scenes, etc., and, on the other hand, have contributed to making an improvement in portability of cameras through miniaturization of films. These have been achieved due to improvements in the art of heightening sensitivity, graininess, resolving power and sharpness. Because increasing the sensitivity of a silver halide photographic material, as has so far been known, necessitates an increase in size of silver halide grains, increasing the sensitivity also causes deterioration of graininess and lowering of the resolving power. On the other hand, miniaturization of films requires an increase in magnification rate upon printing, taking the case of negative films, which brings about deterioration of graininess and resolving power and lowering of sharpness as an inevitable consequence. In order to aim at further increasing sensitivities of films and further miniaturizing films in the future, it is necessary to newly take long strides in these arts.
Known skeleton structures of yellow couplers include those of a pivaloylacetanilide type, a benzoylacetanilide type, a malondiester type, a malondiamide type, a dibenzoylmethane type, a benzothiazolylacetamide type, a malonic ester. monoamide type, a benzothiazolyl acetate type, a benzoxazolyl acetamide type, a benzoxazolyl acetate type, a benzimidazolyl acetamide type, a benzimidazolyl acetate type, and so on. Of these skeleton structures, those of the benzoylacetanilide and the pivaloylacetanilide types are of greater advantage.
Specific examples of the foregoing yellow couplers are described in U.S. Pat. Nos. 2,875,057, 3,265,506, 3,408,194, 3,551,155, 3,582,322, 3,725,072, 4,356,258 and 3,891,445, West German Pat. No. 1,547,868, West German Patent Application (OLS) Nos. 2,219,917, 2,261,361 and 2,414,006, British Pat. No. 1,425,020, Japanese Patent Publication No. 10783/76, and Japanese Patent Application (OPI) Nos. 26133/72, 73147/73, 102636/76, 6341/75, 123342/75, 130442/75, 21827/76, 87650/75, 82424/77 and 115219/77 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), and so on. However, a silver halide photosensitive material which possesses all of the foregoing various properties satisfactorily has not yet been developed.
Hitherto, attempts to enhance graininess and sharpness of a color photoraphic material by devising a usage of couplers have been known. For instance, an art of enhancing the graininess which involves using two couplers differing in coupling reaction rate in two emulsion layers having the same color sensitivity respectively, that is, using a 4-equivalent coupler having a higher coupling reaction rate in the high sensitive emulsion layer, while using a 4-equivalent coupler having a lower coupling reaction rate in the low sensitive emulsion layer, is disclosed in U.S. Pat. No. 3,726,681. However, those couplers provide low color density upon color development. In particular, they manifest markedly lowered color-forming property when the amount of a high boiling organic solvent used for dissolving them is reduced as a method for thinning the emulsion layer, because thinning the emulsion layer is favorable for enhancing sharpness. In addition, the yellow dye image produced from those couplers has a serious defect in that its light fastness is remarkably low.